<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style>
span { display:block; outline: 10px solid yellow; }
</style>
</head>

<body>

<div>
  <div style="display: -moz-inline-grid">
    <span>M</span>
    <span>N</span>
  </div>
</div>

</body>
</html>
